1. Field of the Invention
The present invention relates generally to a clock signal generator for telephone switching systems and the like for generating a redundant clock signal.
2. Description of the Related Art
Based on agreements of the national postal administrations in, for example, West Europe the national digital networks are operated in unsolicited fashion. To avoid losses of information, this unsolicited plesio chronic operating mode requires an extremely precise observation of the prescribed clock frequency as well as a high dependability against outage or interruption of the clock signal. To this end, central clock generators are provided in digital switching centers of the digital network and are guided from a central reference frequency source. To further increase the reliability against the occurrence of an outage of the clock signal, redundant clock generators are provided in the digital switching centers. The redundant clock generators are used to check each other mutually and, when a fault occurs in the active clock generator, the system automatically switches to the other standby clock generator.
In known clock distributor means in digital switching centers, as described in a Telcom Report, 4, (1981) supplement "Digitalvermittlungssystem EWSD", pages 43-48, the distribution of the clock signal to switching matrix array units to be supplied with the clock signals ensues via copper lines, so that a line driver must be provided for every line. Given employment of a copper lines formed of wire pairs that are balanced to ground, moreover, two line drivers per copper line must be provided. In addition to the outlay for components used in the line drivers, power losses which occur, for example, due to the recharging of the copper lines at a clock frequency of 140 MHz that is typical for digital switching centers or as appear in the terminating impedances of the copper lines have a disadvantageous effect. Over and above this, special measures are necessary for these clock frequencies to avoid the emission of or picking up of disturbances from neighboring lines.